1. Field of the Invention
This invention relates to a method of manufacturing high permeability Fe-Ni system alloy, and in particular to a method of manufacturing high permeability Fe-Ni system alloy which omits the hot-rolling step.
2. Description of the Prior Art
High permeability Fe-Ni magnetic alloys are widely used as magnetic shielding materials. For example, such alloys are used to encase magnetic heads and as magnetic baffles for cassette tapes. Of such alloys, in particular, frequent use is made of high nickel permalloys (JIS-PC) and low nickel permalloys (JIS-PB) containing elements such as molybdenum, chromium and copper. While high nickel permalloy possesses high permeability and good resistance to corrosion, a drawback is that it is costly, containing as it does around 80% nickel, which is an expensive element, and the even more costly element, molybdenum. While low nickel permalloy is cheaper, having a nickel content of around 45%, and has a high saturation flux density of 15,000 G, it too has a drawback, which is that its alternating current permeability is much lower than that of high nickel permalloy.
Furthermore, permalloy is usually cast into ingots and hot-rolled one or more times, as required, at a high temperature of 1000.degree. C. or more to obtain the cold-rolled material. However, during this high temperature heating the surface of the ingots or semiprocessed sheet is highly prone to grain boundary oxidation, so that there is a risk that fracturing may occur during the hot rolling. A further problem is that a special need to surface-grind the material increases the processing load and, as a consequence, produces a marked lowering of the yield. These problems, together with the sharp rises in the price of nickel over the past few years, have created a need for a fundamental reappraisal of permalloy manufacturing methods.
One way is to substitute cheaper elements for part of the nickel content. Such a method in which copper is used as the substitute element is disclosed by JP-A 62-5973/1987, JP-A 62-5974/1987, and JP-B Hei 1-53338/1989, among others, while JP-A Hei 1-252756/1989 uses chromium; in each case, however, the manufacturing process is a conventional one using hot rolling.
A method which omits the hot-rolling step is disclosed by JP-A Hei 1-290715/1989. The method of this disclosure, which focusses on grain orientation, one of the factors that determine magnetic properties, includes the steps of direct sheet-casting and cold-rolling of material with a high concentration of (100) grain texture. This promotes the development of a cubic grain structure which is advantageous in terms of magnetic properties, while at the same time the decreased number of processing steps reduces costs.
The present inventors also conducted extensive experiments relating to direct casting of steel sheet as a way of fundamentally improving the manufacturing process. These experiments showed that JP-A Hei 1-290715/1989 was inadequate in terms of ensuring the requisite magnetic properties.
Specifically, the premise of JP-A Hei 1-290715/1989 is that direct casting of sheet will result in a texture with a high concentration of (100) grains. However, the (100) face strength of actual slabs obtained thus was not very high; if anything, the grain texture was randomized. Moreover, it is known that in the case of permalloy PC, as the magnetic anisotropy constant is close to zero almost no effect can be expected, and in fact the magnetic properties tend to be inferior to those of hot-rolled materials.
JIS (Japanese Industrial Standards) divides Ni-Fe system permalloys into two classes: high nickel permalloys containing around 80% nickel and low nickel permalloys containing around 45% nickel. The high nickel permalloys normally include Mo, Cr, Cu and the like. Although the effect of including these components is a well known part of the history of permalloy development, they will be set forth here.
The first high permeability alloy of this type to be invented contained of about 80% Ni and the remainder of Fe. The high permeability of this component system was found to result from the alloy's extremely low constant of crystal magnetic anisotropy K and a constant of magnetostriction .lambda.. It was later found that the addition of around 5% Mo to this alloy reduced its K and .lambda. values to nearly zero, providing an alloy with almost unsurpassable permeability that came to be known as "Supermalloy." However, for obtaining this very high permeability it was necessary to restrict the cooling speed in final annealing (magnetic annealing) to within extremely narrow limits. This prevented the alloy from coming into general use.
It was next found that adding about 5% Cu to the Ni-Mo-Fe alloy made the cooling conditions less strict, and enabled the production of a high-permeability alloy appropriate for wide utilization (falling under the first JIS category mentioned above and known as JIS-PC).
The foregoing and other research regarding various alloying elements for inclusion in Fe-Ni system alloys was conducted between 25 and 35 years ago; it was also found that Cr and the like are effective for obtaining high permeability.
More recently, the extensive dissemination of high fidelity tape records and other audio equipment led to the use of permalloys in magnetic heads. In the early stage of this application, however, the heads were found to suffer heavy wear because of the softness of permalloys based on the composition systems known at the time. As a result of various attempts to overcome this problem, there were invented permalloys improved in anti-wear property by the addition of hardening elements such as Nb, Ti, Ta and V.